1. Field of the Invention
The present invention relates to non-oriented electrical steel sheet used as the material for an iron core of electrical equipment and its method of production. In particular, it relates to non-oriented electrical steel sheet superior in magnetic properties in the rolling direction after stress relief annealing.
2. Description of the Related Art
In recent years, due to the increasing global trend toward energy saving in electrical equipment, the non-oriented electrical steel sheets used as the materials for the iron cores of motors have been required to be further lowered in core loss and increased in magnetic flux density. In general, Si has been added to increase volume resistivity, the grain size of the product has been increased to reduce the core loss, and the hot band annealing and cold reduction have been optimized to increase the magnetic flux density.
On the other hand, as the method for producing small-sized motors, in recent years so-called segment type has been employed in increasing cases. In this method, steel sheet is punched and stacked in segment pieces, wire-wound, and joined to form an arc shaped stator core. The method has the advantages of improved yield of the steel sheet and improved winding packing rate. The method also has an advantage to enable an alignment of a specific direction of the steel sheet good in magnetic properties with for example the direction of teeth where the magnetic flux concentrate, by which an improvement in the motor efficiency can be expected.
As the steel sheet for such segment cores, use of grain-oriented electrical steel sheet with extremely good magnetic properties in the rolling direction may be considered, but the punchability of the sheet is poor and the cost ends up greatly increasing. So there have been almost no cases of its use in such motors but, like with conventional motors, non-oriented electrical steel sheet is being employed. That is, if it is possible to remarkably improve the magnetic properties in a specific direction in non-oriented electrical steel sheet, such sheet should be possible to be an optimal material for a segment type small-sized motor.
As a non-oriented electrical steel sheet for segment cores, for example, Japanese Unexamined Patent Publication No. 2004-332042 discloses a method wherein particularly controlling of a crystal grain size after hot band annealing and the reduction of cold rolling results in the development of a {100}<001> type texture after the final annealing and superior magnetic properties in the rolling direction and the direction vertical to the rolling direction of the surface.
However, in non-oriented electrical steel sheet up to now, the fact is that even in the rolling direction with good magnetic properties (hereinafter called the “L-direction”), the superiority of the magnetic properties over the other directions of the steel sheet is small. Furthermore, recently, there has been a growing need for thin and high Si content high grade sheets for the purpose of reducing a high frequency core loss and there has been the problem that the superiority of the L-direction magnetic properties becomes smaller in such steel sheets.